feat: backend /proxy endpoint for full-res RAW/HEIC display

Adds GET /photos/{id}/proxy that decodes RAW (rawpy), HEIC (pillow-heif),
and TIFF to a cached full-resolution WebP at /data/proxies/{id}.webp.
Web-safe formats (JPEG/PNG/WebP/GIF) pass through to the original to
avoid pointless transcoding. RAW failures fall back to extracting the
embedded JPEG preview. Mirrors the X-Accel-Redirect pattern from the
existing thumb endpoint.

Also fixes GET /photos/{id}/original to return the correct image/jpeg,
image/png, video/mp4, etc. content types instead of always serving
application/octet-stream, so <img> and <video> tags can render the
file inline rather than triggering a download.

Frontend: adds photos.getProxyUrl() helper in services/api.ts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/services/api.ts

@@ -85,6 +85,12 @@ export const photos = {
   getOriginalUrl: (photoId: string) => {
     return `${API_BASE_URL}/photos/${photoId}/original`
   },
+
+  /** Full-resolution display URL. Backend serves the original for web-safe
+   *  formats and a transcoded WebP for RAW/HEIC/TIFF. */
+  getProxyUrl: (photoId: string) => {
+    return `${API_BASE_URL}/photos/${photoId}/proxy`
+  },
 }
 
 // Library API